1. Field of the Invention
The present invention discloses an improved process for oxidizing alkyl aromatic hydrocarbons and/or their partially oxidized intermediates to produce aromatic carboxylic acids. The process involves the liquid phase oxidation in the presence of a catalyst of cobalt-manganese-bromine and alkali metal or alkaline earth metal, in an aliphatic carboxylic acid having 1.about.6 carbon atoms such as acetic acid as a solvent with a feed gas containing oxygen either with or without carbon dioxide. In particular, one or more than one type of alkali metal or alkaline earth metal components are preferably added to the catalyst system, and furthermore, an additional transition metal or lanthanide series metal is introduced to the catalyst system, cobalt-manganese-bromine, when it is deemed necessary.
The rate of the oxidation reaction of an alkyl aromatic substrate was remarkably increased in the present process over the conventional MC-type process (i.e., a liquid phase oxidation reaction using a cobalt-manganese-bromine catalyst). The yield and quality of the carboxylic acid product were also significantly improved by the present process. Thus, terephthalic acid of improved yield and purity is produced by carrying out the oxidation of para-xylene in the presence of a catalyst containing the additional components such as potassium and/or transition metal in the co-presence of carbon dioxide with oxygen, at relatively mild reaction conditions.
With the present invention, highly pure terephthalic acid or isophthalic acid can be produced by oxidizing impurities such as 4-carboxybenzaldehyde, para-toluic acid, 3-carboxybenzaldehyde, and meta-toluic acid contaminated in crude terephthalic acid and crude isophthalic acid, respectively.
2. Description of the Related Art
As discussed below, methods of manufacturing aromatic carboxylic acids are well known and are widely used commercially. For example, a method of manufacturing of aromatic carboxylic acids such as terephthalic acid (TPA), isophthalic acid (IPA), phthalic acid, phthalic anhydride, naphthalene dicarboxylic acid, trimellitic acid, trimellitic anhydride, trimesic acid, pyromellitic dianhydride, 4,4'-biphenyldicarboxylic acid and benzoic acid by oxidizing alkylaromatic compounds or the oxidized intermediates thereof, in the presence of cobalt-manganese-bromine, from such alkylaromatic compounds as para-xylene, para-tolualdehyde, para-toluic acid, 4-carboxybenzaldehyde (4-CBA), meta-xylene, meta-tolualdehyde, meta-toluic acid, 3-carboxybenzaldehyde, ortho-xylene, dimethylnaphthalene, pseudocumene (1,2,4-trimethylbenzene), mesitylene (1,3,5-trimethylbenzene), durene (1,2,4,5-tetramethylbenzene), pentamethylbenzene, hexamethylbenzene, 4,4'-dimethylbiphenyl and toluene is well known (for example, U.S. Pat. Nos. 2,833,816 and 5,183,933). Such aromatic carboxylic acids are used as raw materials for manufacturing polyester after appropriate purification such as hydrogenation, etc. (U.S. Pat. No. 3,584,039). Also, polyester is widely used as a synthetic fiber, film, etc.
There were continuous endeavors to develop a catalyst system with high efficiency and enhanced reactivity to manufacture aromatic carboxylic acids. The newly developed technologies, however, were not practical due to the increase of side reactions, price of catalyst, difficulty of operation, and precipitation of catalyst, etc.
Improvements of the efficiency of the reaction and the catalyst in the manufacturing of aromatic carboxylic acids are very significant because they may improve productivity, quality and cost competitiveness due to the reduction in the reaction time and side reactions. In other words, it is highly desirable to develop a technology to increase the efficiency of the oxidation reaction of alkyl aromatic compounds and the oxidized intermediates thereof by means of an improvement in the catalyst system or other reaction processes.
There were many attempts to increase the efficiency by adding a third metal catalyst to the cobalt-manganese-bromine catalyst system which is the basic catalyst system, to enhance the catalyst efficiency during the manufacturing of aromatic carboxylic acids. The added metals were mainly transition metals, and by adding, for example, hafnium, zirconium, molybdenum, etc., the reactivity therein was increased (U.S. Pat. No. 5,112,992). Further, as an example of an attempt to add an alkali metal component, a catalyst system was used, in which alkali metal components such as lithium, sodium and potassium were added to the cobalt-manganese-nickel-bromine catalyst system, in the presence of two or three types of bromine compounds. There, the method involved manufacturing of terephthalic acid of a monomer grade by a two-step process of oxidation and re-crystallization (WO96/41791). In that method, there is a disadvantage in that the catalyst system is very complicated, since nickel must be added to cobalt-manganese-bromine for the catalyst system and since more than two types of bromine compounds are necessary (both compounds having an ionic bond and those having a covalent bond are needed).
The newly developed technologies, however, were not practical due to the increase of the side reactions, price of catalyst, difficulty of operation, and precipitation of catalyst, etc. even though there were many attempts to develop a catalyst system for aromatic carboxylic acids with high efficiency and enhanced reactivity.
On the other hand, an oxygen containing gas such as air was mainly used as an oxidant during the manufacturing of aromatic carboxylic acids. Carbon dioxide was not used as an oxidant due to its chemical stability. Yet, in the research for improving the process efficiency, there was a case in which chemically stable carbon dioxide, recycled from the reaction vent gas, was injected to the reactor to increase the stability in the process by mitigating the problematic possibilities of explosion due to oxygen when using pure oxygen or gas containing pure oxygen or oxygen enriched gas as an oxidant (U.S. Pat. No. 5,693,856). Nevertheless, the case is not known in which carbon dioxide was added to improve the reaction efficiency.
In summary, the basic oxidation technologies for aromatic carboxylic acids manufacture, especially for TPA manufacture, have been extensively developed. The basic process technology is now approaching a point of diminishing returns, and further major breakthroughs i.e., new catalyst systems, raw materials, and basic unit operations, are not anticipated. The leading PTA (purified terephthalic acid) producers are expected to have greater optimization and energy integration across the entire production complex and more advanced control schemes. However, surpassing the current general expectation, this invention made a remarkable breakthrough to achieve improved catalyst activity and selectivity toward aromatic carboxylic acids, especially for PTA, in the aforementioned catalyst composition under milder oxidation conditions.